The present invention relates generally to servo mechanisms, and more particularly to a servo mechanism for controlling a means having a controllable member in response to a difference, or an error signal, between a signal indicating the power required of the means and an actual speed or feedback signal thereof.
Conventionally, servo mechanisms comprise only one spool valve means and an actuator having a piston which is operatively connected to a means having a controllable member. The piston is slidably disposed within a housing of the actuator in order to thereby define first and second chambers within the housing. Each chamber is hydraulically connected through the spool valve means to a source of pressurized fluid and a reservoir. The spool valve means thus arranged is associated with an input signal of the required power and an actual or feedback signal of said means having the controllable member. Thus the spool valve means has three positions, namely, a first position wherein each chamber is interrupted from the pressurized fluid source and the reservoir, a second position wherein the first chamber is hydraulically connected to the pressurized fluid source while the second chamber is hydraulically connected to the reservoir, and a third position wherein the first and second chamber are hydraulically connected to the reservoir and the pressurized fluid source, respectively. Thus the piston will be stopped or moved in response to the position of the spool valve means. However, this requires four valving portions of the spool valve means and the relative position between these four valving portions must be formed with high accuracy which of course results in comparatively high manufacturing costs.
In order to obviate the drawbacks mentioned above, further conventional servo mechanisms have been proposed wherein a piston has large and small hydraulic pressure receiving areas. The small receiving area is adapted to be exposed to the first chamber which is always connected to a source of hydraulic fluid, and the large receiving area is adapted to be exposed to the second chamber which is connected through a spool valve means to the fluid source and a reservoir. However, such servo mechanisms still require two valving portions of the spool valve means, and thus such servo mechanisms also require high accuracy in construction and will also be comparatively high in cost.
Another conventional servo mechanism comprises a piston having small and large hydraulic pressure receiving areas which are exposed to a first and a second chamber, respectively. The first chamber is always connected to a source of hydraulic fluid while the second chamber is always connected to the fluid source through means of an orifice and is further connected to a reservoir through a spool or poppet valve means. Due to the orifice interposed between the fluid source and the second chamber, however, the speed by which the hydraulic pressure within the second chamber increases will be delayed, so that movement of the piston will also be delayed.